1. Field of the Invention
The present invention relates to an apparatus and method for controlling an inverter and, more particularly, to an apparatus and method for controlling an inverter capable of providing a stable AC output voltage to an induction motor regardless of a variation of a DC input voltage.
2. Description of the Prior Art
In a system having an inverter and driving a motor by using the inverter, an input voltage supplied to the inverter is a DC voltage obtained by rectifying an AC current from a commercial AC power source by a rectifying circuit and then smoothing it by a condenser. It is also called a DC link voltage. The DC input voltage is changed as the voltage of the general AC power is frequently changed according to industrial activity factors (day time during which factories are operated and night time during which factories are not operated) or seasonal factors (the summer season during which air conditioners are actively operated and other seasons).
The change in the DC input voltage may break a current path to the inverter due to an over voltage trip (in case of an over voltage) or bring about a situation that a motor cannot be started (in case of under voltage).
That is, when the input voltage of the inverter is changed, an output voltage is changed at the same ratio, and when the input voltage exceeds a rated voltage where a breaker is tripped, a circuit breaker connected with a power circuit of the inverter is tripped to protect the inverter and the motor to break power supply to the inverter. Conversely, if the input voltage is lower than allowable range of the rated voltage, the motor cannot be started by failing to generate a motor start torque.
Thus, in order to compensate the input voltage lower than the allowable range of the rated voltage, a technique of providing a boost voltage set by a user to the inverter is proposed. Namely, if the DC input voltage is an under voltage, the boost voltage is set as a positive (+) voltage, whereas if the DC input voltage is an over voltage, the boost voltage is set as a negative (−) voltage.
One example of the prior art will be described with reference to FIG. 1. As shown in FIG. 1, the prior art inverter controlling apparatus includes an input voltage detector 1 for detecting a DC input voltage inputted to an inverter; an output voltage ratio computing unit 2 for calculating a ratio of an output command frequency to a reference frequency and determining a ratio of an output voltage to an input voltage; a compensation voltage computing unit 3 for obtaining a compensation voltage value by adding a predetermined boost voltage value to a voltage value obtained by multiplying the ratio provided by the output voltage ratio computing unit 2 to the DC input voltage value provided by the input voltage detector 1 and outputting it; and the inverter 4 for outputting an output voltage of the compensation voltage value received from the compensation voltage computing unit 3 to a motor.
The operation of the prior art inverter controlling apparatus constructed as described above will be explained.
First, the output command frequency of the inverter 4 is outputted from a central processing unit to the output voltage ratio computing unit 2 according to a pre-set program.
Then, the output voltage ratio computing unit 2 calculates a ratio of an output command frequency, e.g., 30 Hz, to a reference frequency, e.g., 60 Hz (Herz), which has been predetermined and stored, determines the result, ½, namely, 50%, as a ratio of the output voltage to the input voltage of the inverter 4, and outputs it.
The input voltage detector 1 detects a value of voltage (also called the DC link voltage as mentioned above) across a condenser (not shown) of an input power circuit (not shown) of the inverter 4 as an input voltage value, and provides it. Herein, the input voltage detector 1 can be substantially formed as a potential transformer.
As above mentioned, in order to compensate the over voltage or the under voltage of the input voltage, the positive (+) or negative (−) boost voltage, which has been previously set and stored in the inverter controlling apparatus by an input unit such as a program loader by a user, can be provided in the compensation voltage computing unit 3 by the central processing unit.
The compensation voltage computing unit 3 calculates a compensation voltage value by multiplying the ratio provided from the output voltage ratio computing unit 2 to the input voltage value provided from the input voltage detector 1, and adds the pre-set boost voltage value to the obtained compensation voltage value to thereby obtain a final compensation voltage value and output it to the inverter 4.
Then, the inverter 4 outputs an AC output voltage according to the compensation voltage value provided from the compensation voltage computing unit 3 to the motor.
However, in the prior art, if a present input voltage becomes smaller than the input voltage when the boost voltage is set, the output voltage of the inverter is accordingly reduced. Then, the start torque is not sufficient to start the motor.
In addition, when the current input voltage becomes higher than the input voltage when the boost voltage is set, the circuit breaker performs the over current trip operation to cut off power supply to the motor.
Meanwhile, in case of a low speed operation, especially when the motor is started, even when the input voltage of the inverter is changed by merely 10%, the current flowing at the motor is changed by more than 10%. Thus, if the present input voltage is changed greater or smaller than the input voltage when the boost voltage is set, the problem of over current trip or the start torque deficiency occurs.
In addition, in order to avoid the problem, in the prior art, the user must re-set the boost voltage value frequently.